研究目的
To develop and demonstrate a microscopic free carrier absorption (FCA) system for depth-resolved carrier lifetime measurements in SiC wafers without requiring cross-sectional cuts, enabling control of carrier lifetime distribution for optimizing high-voltage bipolar device performance.
研究成果
The developed microscopic FCA system successfully enables depth-resolved carrier lifetime measurements in SiC without cross-sectional cuts. For uniform samples (CMP), lifetimes increase with depth due to reduced surface recombination, while for samples with a recombination-enhancing layer (wREL), lifetimes decrease with depth, correlating with the layer's presence. This system is an effective tool for observing and controlling carrier lifetime distributions in SiC wafers, aiding the optimization of high-voltage bipolar devices. Future work could focus on improving depth accuracy and exploring lower excitation conditions.
研究不足
The system's depth resolution is limited by the focusing optics and refractive index effects, with measurement positions approximated as three times the sample movement. High excitation conditions (carrier concentration ~1e19 cm-3) may dominate by Auger recombination, potentially masking other recombination mechanisms. Surface effects near the sample surface could influence measurements, and the technique requires precise alignment and control of laser overlap.
1：Experimental Design and Method Selection:
The study employs a free carrier absorption (FCA) technique with overlapping probe and excitation laser lights to achieve depth resolution. The system uses a pulsed YAG laser for excitation and a continuous wave laser for probing, focused by an objective lens to localize the measurement area.
2：Sample Selection and Data Sources:
Two n-type 4H-SiC samples were used: a CMP sample (free-standing epitaxial film, doping ~1e15 cm-3, thickness 100 μm) and a wREL sample (epitaxial film with n- drift layer and n+ buffer layer, doping ~1e16 cm-3 and ~1e18 cm-3, thicknesses 9.4 μm and 11.1 μm).
3：4 μm and 1 μm).
List of Experimental Equipment and Materials:
3. List of Experimental Equipment and Materials: Equipment includes a 355 nm pulsed YAG laser, a 405 nm continuous wave laser, an objective lens (numerical aperture 0.5, focal length 5 mm, aperture 5 mm), silicon photodiode detector, low-pass filter for 355 nm, band-pass filter for 405 nm, and a sample stage for precise movement.
4：5, focal length 5 mm, aperture 5 mm), silicon photodiode detector, low-pass filter for 355 nm, band-pass filter for 405 nm, and a sample stage for precise movement.
Experimental Procedures and Operational Workflow:
4. Experimental Procedures and Operational Workflow: Both lasers are focused on the sample with incident angles of 25° and -25°, and the transmitted probe light is detected. The sample is moved in 0.5 μm intervals toward the objective lens to vary the measurement depth. Data is collected on FCA signal decay over time at different depths.
5：5 μm intervals toward the objective lens to vary the measurement depth. Data is collected on FCA signal decay over time at different depths.
Data Analysis Methods:
5. Data Analysis Methods: The 1/e lifetime is defined as the decay time from peak to 1/e of the FCA signal. Depth profiles are analyzed by correlating sample movement with actual depth (approximately three times the movement due to refractive index effects), and results are interpreted in terms of surface recombination and layer-specific lifetimes.
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